Ink-jet printing is a technique in which ink droplets are ejected through very fine nozzles toward a recording medium and deposited directly onto the recording medium, to thereby provide the medium with characters or images. This technique is very prevalent, since it has various advantages, in that full-color printing is readily and inexpensively achieved, plain paper can be used as a recording medium, and a printing head does not come into contact with an object to be printed.
Recently, an ink containing a pigment serving as a coloring material has been widely used for imparting weatherability or water resistance to a printed object. In order to prevent bleeding of the ink or to improve fixation of the pigment onto a paper sheet, a resin emulsion or the like is added to the ink.
Patent Document 1 discloses an aqueous pigment dispersion containing a pigment, a hydrophobic-group-containing cationic polymer, and a hydrophobic-group-containing anionic polymer, which dispersion is provided for improvement in, for example, dispersion stability.
Patent Document 2 discloses an ink for ink-jet printing containing a cationic surface-modified self-dispersible pigment, anionic resin particles exhibiting a zeta potential of −15 mV or less at a pH of 6, and an aqueous medium, which ink is provided for improvement in, for example, optical density.
Patent Document 3 discloses an ink containing a first pigment (i.e., an anionic or cationic self-dispersible pigment), a second pigment (i.e., a pigment which can be dispersed in an aqueous medium by a polymer dispersant), a polymer dispersant having the same polarity as the first pigment, and a nonionic polymer dispersant, which ink is provided for improvement in, for example, image density.
Patent Document 4 discloses a method for forming an ink-jet ink composition, the method including a step of combining an emulsion of an emulsifiable polymer resin in an anionic medium with a pigment and a cationic surfactant, and aggregating particles in the resultant mixture to a desired particle size.
Patent Document 5 discloses a colorant for an ink-jet ink, the colorant containing composite powder particles formed of extender particles whose surfaces are coated with a paste on which an organic pigment layer is uniformly deposited.
Patent Document 6 discloses (1) a dispersible coloring material for ink-jet printing containing a coloring material, and chargeable resin pseudo fine particles having a size smaller than that of the coloring material, the particles being fixed or fused onto the coloring material, wherein the coloring material itself has a surface charge; and (2) a process for producing a dispersible coloring material, wherein radically polymerizable monomers are subjected to aqueous precipitation polymerization in an aqueous dispersion solution in which a water-insoluble coloring material having a polar group bonded to its surface is dispersed, to thereby form chargeable resin pseudo fine particles fixed or melt-adhered to the surface of the water-insoluble coloring material.    Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 2008-239961    Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. 2003-306624    Patent Document 3: Japanese Patent Application Laid-Open (kokai) No. 2000-239589    Patent Document 4: Japanese Patent Application Laid-Open (kokai) No. H09-124985    Patent Document 5: Japanese Patent Application Laid-Open (kokai) No. 2003-55591    Patent Document 6: Japanese Patent Application Laid-Open (kokai) No. 2006-37087